Supercite

It's not a thing, it's an Emacs Lispprogram. It allows you to cite parts of newsposts or emails even if they already contain several generations of citations. It can re-flow text and produces very readable output. It is best used with the Gnus newsreader, and for further enjoyment of Gnus, I would recommend BBDB. Supercite is available as the file "sc.el".